Electrical filter



May 3, 1932. G. B. CROUSE ELECTRICAL FILTER Filed Dec. 29. 1927 2 SheetsSheet Elite": @130 Patented May 3, 1932 UNITED STATES PATENT OFFICE GEORGE B. CROUSE, OF WOODCLIFF, NEW JERSEY, ASSIGNOR TO CONNER CROUSE COR- PORATION, OF NEW YORK, N. Y.,

A CORPORATION 035 NEW YORK ELECTRICAL FILTER Application filed December 29, 1927.

This invention relates to an electrical filter for removing alternating components from rectified or pulsating current, and more particularly to a filter capable of suppressing alternating components to the degree required for the satisfactory operation of radio receivers.

The filter network is of the same general type and has the same suppression characteristic as the filter described in my copending application, Serial No. 680,096, filed December 12, 1923, which became Patent No. 1,804,859, May 12, 1931.

An object of the invention is to provide asimple and economical filter which will effect a substantially complete suppression of all alternating components present in a rectified current supply. A further object is to provide a filter of the Wheatstone bridge type for use between a source of rectified current and the filament circuits, and which filter requires no condensive reactance in the bridge arms. A further object is to provide a filter for use in a power supply device, which filter includes a VVheatstonebridge network balanced at the frequency of the most disturbing alternating component of the rectified current, and additional circuit elements for eliminating components of frequencies other than that for which the network is balanced.

These and other objects of the invention will be apparent from the following specification when taken with the accompanying drawings in which Fig. 1 is aschematic diagram of one embodiment of the invent-ion, and

Figs. 2 and 3 are vector diagrams drawn for open circuit load condition of the bridge filter.

In the drawings, the numeral 1 identifies a source of alternating current for energizing the primary winding 2 of atransformer having a pair of secondaries 3, and a secondary 4 which supplies the filaments of a pair of rectifiers The secondaries 3 and the rectifiers 5 are connected in the usual way to effect full wave rectification, and the usual bufling condenser 6 is preferably used to eliminate ra equency radiation. A posi- Serial No. 243,409.

tive lead 7 and negative lead .8 supply energy to the filter network in the form of rectified current.

It is to be noted that the details of this converter system for securing rectified current from the alternating current source form no part of the present invention. The construction of any particular filter is of course dependent upon the frequencies and magnitudes of the alternating components present in the rectified current, but the same circuit arrangement may be employed with full or half-wave rectified alternating current or with the pulsating direct current output of a generator.

Thefour arms A, B, C, D of the Wheatstone bridge network are dissimilar impedances which provide parallel paths between the rectified current leads 7 and 8. The arm A comprises an inductance 9 wound upon an iron core 10, and arm B comprises the reverse mutual inductance 11 on core and a resistance 12. Arm C comprises resistance 13 and the inductance 14 wound on an iron core 15, and arm D comprises the reverse mutual inductance 16 on core 15.

The leads 17, 18 from the audion circuits are connected between the junctions of arms A, B and arms C, D. An inductance 19 on an iron core 20 is preferably arranged in series with the load to aid in the suppression of frequencies for which the bridge is not perfectly balanced. When additional suppression of certain frequencies is desirable, an auxiliary circuit is provided which comprises a coil of a few turns 21 coupled to the series coil 19, and connected across the inductance 14 through acondenser 22.

The operation of the bridge network will be best understood from the vector diagram drawn for open circuit load condition. Fig. 2 represents voltages of the frequency for which the bridge is balanced while Fig. 3 represents voltages of a higher frequency.

In the right hand diagram of Fig. 2, the alternating voltage between the rectified current leads 7 8 is represented by vector 30. The voltage across the bridge arm A, which is formed by purely inductive impedance 9, is represented by vector 31. Two voltages will be present in arm B since that arm includes inductance 11 and resistance 12. The vector 32 indicates the voltage across the inductance 11 and is in line with and opposite to the voltage 31 across inductance 9, while the vector 33 represents the voltage across resistance 9. The total voltage across arm B is then represented by vector 34 which is the vector sum of vectors 32 and 33.

Turning now to the left hand diagram, the total voltage across arms O and D will again be vector 30. The voltages in arm C, i. e., across the resistance 13 and inductance 14, are at right angles to each other and are rep resented by vector 35 across inductance 14 and vector 36 across the resistance 13. These two add to give the vector 37 which, for bridge balance, is to be the same in magnitude and direction as vector 31. The impedances forming bridge arm C are so pro- 'portioned that in the vector diagrams the vector 35 (voltage across inductance of arm C) is in line with the vector 34 of the right hand diagram (voltage across arm B). Vector 38 represents the voltage across inductance arm D, and this vector is in line with and opposite to the vector 35. By a proper choice of the number of turns in inductance 16, the magnitude of vector 38 will be equal to that of vector 34. When this condition is satisfied. the bridge is balanced for the particular frequency considered in the preparation of the vector diagrams, and a direct current load circuit connected across the junctions of arms A, B, and B, C, respectively, will have an infinite impedance at that frequency.

The bridge network will, of course, be unbalanced at other frequencies and an idea of the degree of unbalance may be most readily gained from vector diagrams, for a higher frequency, of the voltages across the four arms. In Fig. 3, diagrams similar to those of Fig. 2 are presented, homologous vectors being identified with corresponding numbers bearing the prime mark. The voltages represented in. Fig. 3 are of double the frequency for which the bridge was balanced. It will be noted that vectors 34 and 38 are substantially parallel at the higher frequency. thus indicating that the phase relationship of voltages across arm B and arm D remains subinto the load circuit a reverse alternating voltage which is controlled in phase and magnitude by the condenser 22.

The resistors 12 and 13 are given such values that only a moderate amount of direct current is by-passed around the load circuit.

The bridge is balanced for the frequency which has the most disturbing effect upon the load circuit, and in some instances the most disturbing frequency is not that of the alternating component of greatest magnitude. For example, in employing rectified current from a 25 cycle power line to energize a radio receiver, the alternating component of greatest magnitude is 50 cycles when full wave rectifiers are used. Components of this fre quency are substantially suppressed in the radio receiver and loud speaker, but the third harmonic of this frequency falls within the audio range which is highly amplified in the receiver and is not suppressed by the reproducer. Althougl 150 cycle components are present in the rectified output in quantity of an order materially lower than that of the 50 cycle component, the most economical construction of the filter mesh is obtained when the bridge is balanced for alternating components of 150 cycle.

The suppression characteristic of the network is distinguished from that of networks of the type including series inductance and shunt capacities by the fact that the maximum suppression is presented to frequencies at the lower range of frequencies passed by an alternating current rectifier.

I claim:

1. A filter mesh for use between a source of rectified current and an audion circuit,said mesh comprising a plurality of impedances arranged as the four balancing arms of a lVheatstone bridge having conjugate pairs of junction points serving as input terminals and output terminals across which said source and said circuit, respectively, may be con-- nected, each of said arms including an inductive impedance, and the inductances in the two bridge arms terminating at each output terminal being, as viewed from said input terminals, oppositely wound on a common core.

2. In apparatus for energizing an audion circuit from a source of rectified current, a pair of alternating current paths adapted to be connected in parallel across said source, each of said paths including a pair of inductances having reverse mutual inductance when traversed by current from said source, and the junctions of the inductances of each path serving as terminals for said audion circuit.

3. The invention as set forth in claim 2, wherein each of said paths includes a high resistance, the resistances of the two paths being oppositely arranged in said paths to substantially exclude direct current from portions thereof.

4. In apparatus for energizing an audion circuit from a source of rectified current, a network comprising a plurality of impedances arranged as the four balancing arms of a Wheatstone bridge across which said source and circuit may be connected as conjugate arms, and means for impressing upon the audion circuit an alternating voltage to neutralize components of frequencies for which the bridge arms are not balanced.

5. The invention as set forth in claim 4, wherein said means comprises a series inductance-capacity circuit connected between one bridge arm and a lead to the said audion circuit.

6. The invention as set forth in claim 4 wherein said means comprises an inductance serially connected between the bridge network and the terminals of the audion circuit, and a circuit inductively coupled to said inductance and connected to one bridge arm.

7. An electrical filter of the type including an impedance network for use between an audion circuit and a current supply comprising a source of 25 cycle alternating current and a current rectifier, characterized by the fact that the impedances of said network are so proportioned that the network effects a maximum suppression at a frequency of sub-- stantially cycles.

In testimony whereof, I afiix my signature.

GEORGE B. GROUSE. 

